Within the microwave range and higher frequency ranges various types of antennas are used. The slot antenna is such a type of antenna where slots constitute the radiating elements of the antenna. The radiating elements/slots are fed by means of various forms of transmission lines such as waveguides, strip-lines etc. Among other things, the characteristics of the antenna are affected by the location and the design of the slots and the radiation pattern of the antenna is defined by the interaction between the slots.
In its most simple form a slot antenna may consist of a waveguide in the wide side of which a number of longitudinal slots is arranged. The length of the slot is .apprxeq..lambda..sub.0 /2 where .lambda..sub.0 is the wavelength in free space and the slots are located with a pitch of .lambda..sub.g /2 where .lambda..sub.g is the wavelength in the waveguide.
In this type of slot antennas, the feeding of the slots is controlled by their being displaced sideways from the waveguide center line. Alternating sideways displacement results in the 180.degree. phase correction which is necessary for the slots to radiate with the same mutual phase in spite of the location at the distance .lambda..sub. /2.
An advantage with the slot location is that no grating lobes are generated because the distance between the slots is less than one wavelength.
However, in certain applications an antenna with the opposite direction of polarization is wanted. This is possible with transversal slots, i.e. slots which are placed across the longitudinal direction of the waveguide. However, in this case the earlier mentioned possibility to switch the phase by sideways displacement is lacking. The transversal slots must therefore be arranged with the mutual distance .lambda..sub.g. This results in considerable grating lobes because normally .lambda..sub.g &gt;.lambda..sub.0 (typically .lambda..sub.g .apprxeq.1.4 .lambda..sub.0).
The grating lobes can be suppressed in various ways. For example, in the Swedish patent application SE 9000959-8 a method to suppress grating lobes by means of baffles is described. Another way to achieve the same purpose is to fill the waveguide with a dielectric material with .epsilon.&gt;2. Both the methods have their limitations and are therefore sometimes less suitable.
Transversal slots in waveguides also have a high equivalent impedance (R). For normal height of the waveguide, R/Z.sub.0 .apprxeq.1 (where Z.sub.0 is the characteristic impedance of the waveguide) is valid, but for so-called half-height twice that value, i.e. R/Z.sub.0 .apprxeq.2, is achieved. As the antenna has several slots in the same waveguide, the total load impedance will be very high and a transformation of the input impedance will be necessary. Besides the difficulty with the matching, the high slot impedance will cause the bandwidth of the antenna to be limited.